1. Field of the Invention
The present invention relates to a turbo vacuum pump, and more particularly to an oil-free turbo vacuum pump which is capable of evacuating gas in a chamber from atmospheric pressure to high vacuum.
2. Description of the Related Art
Conventionally, in a semiconductor fabrication apparatus or the like, turbo vacuum pumps have been used for evacuating gas in a chamber to develop clean high vacuum (or ultra-high vacuum). These turbo vacuum pumps include a type of vacuum pump in which a turbo-molecular pump stage, a thread groove pump stage and a vortex pump stage are disposed in series in a pump casing having an intake port and a discharge port, and a main shaft to which rotor blades of these pump stages are fixed is supported by a hydrostatic gas bearing, a type of vacuum pump in which multiple centrifugal compression pump stages are disposed in a pump casing having an intake port and a discharge port, and a main shaft to which rotor blades of these pump stages are fixed is supported by a radial gas bearing and a thrust gas bearing, and other types of vacuum pumps. In this manner, the main shaft is supported by the gas bearing without using a rolling bearing to construct an oil-free turbo vacuum pump which does not require oil in the entirety of the pump including gas passages and bearing portions.
The turbo vacuum pump in which the turbo-molecular pump stage, the thread groove pump stage and the vortex pump stage are combined with the hydrostatic gas bearings is disclosed in Japanese laid-open patent publication No. 2002-285987. This turbo vacuum pump is capable of compressing gas from ultra-high vacuum to atmospheric pressure. In this turbo vacuum pump, vortex flow blades (circumferential flow blades) of the vortex pump stage are blade elements which are capable of compressing gas to atmospheric pressure, even if a blade clearance is wide. The vortex flow blade of the vortex pump stage comprises rotor blade parts formed radially at an outer circumferential portion of a rotating circular disk, annular recesses (flow passages) which surround the rotating circular disk having the rotor blade parts, and a communicating passage for allowing the vertically adjacent flow passages to communicate with each other. However, the vortex pump stage has disadvantages that a volume of the blade element is large because the flow passages for surrounding the rotating circular disk above and below are required. Further, gas is drawn in from a single communicating passage (intake port) provided at the flow passage, compressed in a circumferential direction, and discharged from a communicating passage (discharge port) communicating with the adjacent flow passage. Therefore, the vortex pump stage has disadvantages that evacuation velocity (evacuation capacity) is small. Furthermore, because the rotating circular disk having a lot of rotor blade parts radially formed is rotated in an atmospheric pressure range, a large operating power is required. In addition, the vortex pump stage has structural disadvantages that stationary-side structure having the flow passages and the communicating passage is complicated.
On the other hand, the turbo vacuum pump in which the centrifugal compression pump stages are combined with the gas bearings is disclosed in Japanese laid-open utility model publication No. 1-142594. This turbo vacuum pump is capable of compressing gas from low vacuum range to substantially atmospheric pressure. In this turbo vacuum pump, the thrust gas bearing is disposed at the discharge port side, and a rotating thrust disk of the thrust gas bearing is placed axially between a stationary upper disk and a stationary lower disk. This turbo vacuum pump has disadvantages that the number of parts is large because the centrifugal compression pump stage and the gas bearing are discrete structures. Because the centrifugal compression pump stage and the gas bearing are discrete structures, it is difficult to make the blade clearance of the centrifugal compression pump stage minute.
Further, as a turbo vacuum pump, there is a vacuum pump in which multiple evacuation pump stages are disposed in a pump casing having an intake port and a discharge port, rotor blades in the multiple pump stages are composed of ceramics, and a main shaft for supporting the ceramic rotor blades is composed of a metal having a small coefficient of linear expansion.
Since there is a small blade clearance between the rotor blade and the stator blade in the vacuum pump, heat is generated in the process of compressing gas to increase a temperature of the blades. Therefore, an example in which ceramics are used to construct multistage rotor blades as a material having a small coefficient of linear expansion and a large specific strength is disclosed in Japanese laid-open patent publication No. 5-332287. In this example, a main shaft is composed of a material having a small coefficient of linear expansion so that the difference between the coefficient of linear expansion of the ceramic rotor blade and the coefficient of linear expansion of the main shaft is not more than 5×10−6/°C.
However, in the case where martensitic stainless steel is used as a material for the main shaft, the coefficient of linear expansion of the martensitic stainless steel is about 10×10−6/°C., and the difference between the coefficient of linear expansion of the martensitic stainless steel and the coefficient of linear expansion of silicon nitride ceramics (3×10−6/°C.) as high-strength ceramics used for a rotor is 7×10−6/°C. If austenitic stainless steel is used as a material for the main shaft, the difference between the coefficient of linear expansion of the silicon nitride ceramics (3×10−6/°C.) and the coefficient of linear expansion of austenitic stainless steel (17×10−6/°C.) becomes much larger. Therefore, in the prior art (Japanese laid-open patent publication No. 5-332287), it is necessary for a material of the main shaft to select a material having a high Young's modulus in consideration of a small coefficient of linear expansion and a large natural frequency of a rotating member, resulting in increased cost.
On the other hand, the centrifugal compression pump stage of the turbo vacuum pump disclosed in Japanese laid-open utility model publication No. 1-142594 comprises rotating disks and stationary circular disks which are alternately disposed.
FIG. 28 is a cross-sectional view showing the centrifugal compression pump stage disclosed in Japanese laid-open utility model publication No. 1-142594. As shown in FIG. 28, stationary circular disks 2a, or 2b, are axially positioned and stacked using cylindrical spacers 2a2 or 2b2. Impellers 1a1 or rotating disks 1b1 are formed integrally with a main shaft 1.
In the centrifugal compression pump stage of the vacuum pump disclosed in Japanese laid-open utility model publication No. 1-142594, the stationary circular disks 2a1 or 2b1 are axially positioned and stacked using the cylindrical spacers 2a2 or 2b2, and the impellers 1a1 or the rotating disks 1b1 are formed integrally with the main shaft 1. Specifically, the number of parts is large because blade elements and spacer elements as a stationary assembly are discrete parts. Further, since the main shaft 1 and the rotating disks 1b1 as a rotating assembly are an integral structure, it is difficult to raise axial dimensional accuracy and geometric tolerance accuracy in each stage.